The present invention relates to a microwave seal for the cavity of a microwave oven for heating of food.
Microwave ovens are usually provided with a seal round the opening of the oven cavity to prevent microwave radiation from leaking out of the oven. The seal must be efficient since authorities in a plurality of countries have adopted low limit values of how much microwave radiation is allowed to leak out of the oven cavity.
A microwave oven usually has an oven cavity which is accessible through a door which at the same time constitutes one of the walls of the cavity. As a microwave seal, use has previously been made of, for instance, a sealing cavity which as a rule is placed in the door and which surrounds the opening to the oven cavity. The sealing cavity has a depth corresponding to a quarter of a wavelength of the microwaves to function as a short circuit for the microwaves. An alternative microwave seal is disclosed in U.S. Pat. No. 4,471,194 and is based on two sealing cavity spaces being arranged in parallel in order to further decrease the leakage of microwave radiation out of the oven cavity. The cavity spaces are adjusted so that the leakage is minimised at 2.45 GHz.
The microwave source in a microwave oven usually emits microwaves in the frequency range 2.4 GHz to 2.5 GHz in dependence on, among other things, the load in the oven. It is therefore necessary to achieve good damping in all this frequency range. However, the damping is sensitive to the dimensioning of the sealing cavities.
Microwave ovens are generally fitted with a switch-off means which ensures that the microwave source is switched off as the door is opened. However, the door must in reality be opened somewhat before the microwave source is switched off. Also a small degree of opening of the door results in a change of the electromagnetic properties. A problem in prior-art microwave seals thus is that the leakage of microwaves is sensitive even to a small variation of the space between the door and the marginal area round the opening of the oven cavity.
Another problem is that it may be difficult to minimise in a simple way the leakage of microwaves at the corners of the door since the geometry in the corners is different from that at the sides of the door.
One more problem of the prior art sealing solutions is that it may be difficult to adjust the microwave seal to other irregularities adjacent to the opening, such as hinges and catches.
A main object of the present invention is to provide a microwave oven having a microwave seal, the damping of which is less sensitive to the dimensioning than prior-art microwave seals.
A further object of the present invention is to provide a microwave oven having a microwave seal, the damping of which is less sensitive to the changes in geometry that arise as the door is opened.
One more object of the present invention is to provide a microwave oven having a microwave seal, which can easily be adjusted to different geometries.
A further object of the present invention is to provide a microwave oven having a microwave seal, which can easily be adjusted to different irregularities adjacent to the opening.
These objects are achieved by a device and a method having the features defined in the claims.
For the leakage of microwaves out of a microwave oven to be smaller than the permissible values, sufficient damping of the microwaves in the microwave seal of the oven cavity opening thus is necessary. The starting point of the invention has been that the damping of leaking microwaves in a microwave oven has a maximum at a certain frequency when using a microwave seal having a single cavity according to prior-art technique. The frequency for the maximum depends on the electric length from the oven cavity to the base of the cavity. The microwave seal is conventionally designed so that maximum conforms with the centre frequency of the microwaves.
When using a microwave seal with a single cavity, the desirable damping, however, is achieved in a narrow frequency range only. When opening the door, the electric path from the oven cavity to the sealing cavity changes, and the damping achieves a maximum for another frequency. The change of the maximum of the damping means that the leakage can be too great for certain frequencies at which the microwave source emits microwaves.
A basic idea of the invention is to provide a damping curve that is wide in the frequency plane. This is achieved by using a microwave seal which has a damping of microwaves with maxima at different frequencies, preferably at a frequency slightly above or slightly below the centre frequency round which microwaves are emitted from the microwave source. Thus, the microwave seal is deliberately adjusted so that the damping is not maximised for the centre frequency of the microwaves.
According to one aspect of the present invention, use is made of a microwave seal with two parallel sealing cavities, the electric length of each sealing cavity from the oven cavity being slightly above or slightly below a quarter of a wavelength of the microwaves at the centre frequency. This results in a damping curve as a function of the frequency which has a minimum between two maxima.
In a preferred embodiment, use is made of a microwave seal with two parallel sealing cavities, the electric length from the oven cavity to the base of the sealing cavities being slightly above or slightly below one quarter of a wavelength of the microwaves at the centre frequency. This results in a damping curve as a function of the frequency which has a minimum between two maxima.
By a quarter of a wavelength is meant in the following a quarter of a wavelength of the microwaves at the centre frequency.
The electric length is expressed in the number of wavelengths and is defined for a given frequency. The electric length of, for instance, a waveguide corresponds to the number of wavelengths contained in the waveguide.
The electric length of a cavity can also be changed, for instance, by a material, which has a relative permittivity greater than one, being arranged in the sealing cavity.
The fact that the electric length is a quarter of a wavelength of the microwaves is equivalent to the fact that the natural frequency equals the microwave frequency in the case where the cavity is terminated with a short circuit for the microwaves.
By the damping at the local minimum being made sufficiently great, good damping is obtained over a wide frequency spectrum while at the same time the damping is relatively insensitive to, for instance, the door being slightly opened.
An advantageous choice of the natural frequencies of the sealing cavities is that they are between 50 and 600 MHz, preferably between 150 and 300 MHz, above and respectively below 2.45 GHz, which is the centre frequency of the microwave source.
The microwave seal is preferably placed on the edge portion of the door, the openings of the cavities being directed towards the marginal area round the opening of the oven cavity when the door is closed. Alternatively, the microwave seal is placed round the opening, the openings of the cavities being directed towards the door when closed.
It is particularly advantageous to use a microwave seal which surrounds the opening of the oven cavity and which has the form of two cavities separated by a partition. The partition can then be made separately and then easily be mounted in place, for instance, by welding.
According to one aspect of the invention, the microwave sealing means is formed as a groove which is divided into two cavities by means of a partition. The groove has two essentially opposite side walls. Preferably the partition is provided with damping-controlling projections which are arranged on the partition. The projections are arranged in at least two sets, one set being directed essentially to one side wall and the other set being directed essentially to the other side wall. The two sets of projections affect the cavity length of one cavity each. It is advantageous to employ said partition with projections also in the case where the microwave seal is designed such that the electric length from the oven cavity to the base of the sealing cavity is the same for both sealing cavities.
The partition preferably has the same height as the groove, so that the upper edge of the partition is on a level with the upper edges of the groove.
The projections preferably extend from the upper edge, but alternatively they can extend a small distance down from the upper edge of the partition.
Owing to the projections on the partition being directed opposite ways, the electric lengths of the cavities can be varied independently of each other by varying the length of the projections transversely of the partition.
It is advantageous to design the partition so that the projections are arranged alternatingly directed towards one side wall and towards the other side wall. In this case, only one projection thus extends from each part of the partition. A satisfactory dividing of the cavities is obtained even if less than half the length of the partition is provided with projections.
The partition with projections according to the invention can easily be produced starting from a strip-shaped metal sheet. Then the microwave seal can advantageously be made by a method comprising the steps of
forming a groove round the opening of the oven cavity,
providing a strip-shaped metal sheet which has two longitudinal edges,
making slots from one longitudinal edge to a certain depth therefrom, thereby producing projections,
bending the projections, made between the slots, away from the plane of the metal sheet one way and the other in a predetermined pattern, and
arranging the metal sheet in the groove, so that the groove in the longitudinal direction is divided into two parallel cavities, the lengths of the projections transversely of the metal sheet being adjusted so that the cavities obtain the desired electric length and preferably lengths which correspond to slightly more or slightly less than a quarter of a wavelength of the length of the microwaves at the centre frequency.
According to a further aspect of the invention, the projections have a transversely directed inner part adjacent to the partition and an outer part fixed, angled preferably downwards, to the inner part.
Two or more juxtaposed projections can be directed the same way, but preferably the projections are arranged alternatingly in different directions.
The sum of the distance between two projections directed the same way and the width of one of the projections in the direction of the partition is preferably smaller than a quarter of a wavelength.
In the case where the projections are alternatingly bent in different directions, the distance between two projections which are bent the same way is advantageously greater than 5 mm and preferably greater than 10 mm. This facilitates the mounting of the partition in the groove by means of welding.
The partition is preferably formed as a whole piece, but can also be slotted from the edge provided with projections down towards the base of the groove.
In a preferred embodiment, the projections are rectangular, but also other shapes of the projections are possible. Alternatively the projections can be formed as triangles or rectangles with rounded corners.
The partition preferably consists of a homogeneous metal sheet, but may also consist of, for instance, a metal network moulded into plastic.
It goes without saying that the above aspects can be combined in the same embodiment.
In the following, detailed embodiments of the invention will be described with reference to the Figures.